1. Field of the Invention
The present invention relates generally to an X-ray image detector system. More specifically, the invention relates to an X-ray image detector system for use in an X-ray diagnosing system for medical use.
2. Description of the Related Art
In recent years, in the field of medical treatment, the database of medical data for patients makes progress in order to rapidly and precisely carry out medical treatment. Patients usually receive diagnostics of a plurality of medical treatment facilities. In such a case, if there are no data of other medical treatment facilities, there is some possibility that medical treatment can not be precisely carried out. As an example, there is a problem of medicines or drugs. It is required to take account of drugs administered in other medical treatment facilities to administer appropriate drugs to carry out medical treatment.
It is also required to make database for image data of radiography. In accordance with this database system, it is desired to digitize X-ray images. In an X-ray diagnosing system for medical use, a silver halide film is conventionally used to detect an image. In order to digitize this diagnostic data, it is required to scan the film by a scanner after developing the film, so that it takes a great deal of time. Recently, there is realized a system for directly digitizing an image using a CCD camera having a size of about one inch and an image intensifier tube. However, when an image of, such as a lung, is detected by this system, it is required to provide an optical system for condensing light to detect an image of a region of about 40 cm xc3x9740 cm, so that there is a problem of increasing the system size. There is also a problem in that resolution decreases due to the distortion of an optical system.
In order to solve these problems, there is proposed a flat-panel X-ray detector of an indirect conversion system using a thin film transistor (which will be hereinafter referred to as a xe2x80x9cTFTxe2x80x9d) having an active layer of an amorphous silicon as a switching element (see, e.g., U.S. Pat. No. 4,689,487).
FIG. 8 shows a circuit construction of this flat-panel X-ray detector, and the operation thereof will be described below.
This flat-panel X-ray detector is a detector of an indirect conversion system for converting an incident X-ray into luminescent light by means of a phosphor or the like to change the converted light to an electric charge by means of a photoelectric transfer film of each pixel (picture element). This flat-panel X-ray detector has pixels e1,1, . . . , em,n arranged in the form of an array wherein hundreds to thousands pixels are arranged on each side. Each element ei,j (i=1, . . . , m, j=1, . . . , n) has a TFT 701, a photoelectric transfer film 702 and a pixel capacity 703. The photoelectric transfer film 702 and the pixel capacity 703 are connected in parallel. To one end thereof, a negative bias voltage is applied by means of a power supply 704, and the other end is connected to one of the source and drain of the TFT 701. The other end of the source and drain of the TFT 701 is connected to a signal line 705, and the gate of the TFT 701 is connected to a scanning line 706. The on/off of the TFT 701 is controlled by a scanning line driving circuit 707. The terminal of the signal line 705 is connected to an amplifier 710 for signal detection via a switch 709 controlled by a signal line control circuit 708.
If X-rays are incident on the flat-panel X-ray detector, the phosphor irradiated with the X-rays emits light, and the emitted light is converted into an electric charge by means of the photoelectric transfer film 702, so that the electric charge accumulates in the pixel capacity 703. When one scanning line 706 is driven by the scanning line driving circuit 701 so that all of TFTs 701 connected to the scanning line 706 are turned on, the accumulating charge is transferred to the amplifier 710 via the signal line 705. Then, the electric charge for each pixel is inputted to the amplifier 710 by means of the switch 709 to be converted to dot sequential signals capable of being displayed on a CRT or the like. The quantity of electric charge varies in accordance with the quantity of light being incident on each pixel ei,j (i=1, . . . , m, j=1, . . . , n), so that the amplitude of output of the amplifier 710 varies.
The flat-panel X-ray detector of the indirect conversion system shown in FIG. 8 can directly form a digital image by the A/D conversion of the output signal of the amplifier 710. Moreover, it is possible to produce a pixel region of a thin and large-screen by the array of the TFTs 701.
There are other flat-panel X-ray detectors of a direct conversion system for directly converting X-rays being incident on pixels into an electric charge.
The flat-panel X-ray detector of this direct conversion system has no phosphor. At this point, the flat-panel X-ray detector of the direct conversion system is different from that of the above-described indirect conversion system. In addition, in the flat-panel X-ray detector of the direct conversion system, the magnitude of a bias applied to a photoelectric transfer film or an X-ray-to-charge converting film is different from that in the indirect conversion system.
In the case of the indirect conversion system, a bias of several volts to over ten volts is applied to the photoelectric transfer film. When fluorescence enters the photoelectric transfer film, the electric charge accumulates in the pixel capacity provided in parallel to the photoelectric transfer film in each pixel. In this case, the voltage applied to the pixel capacity is a bias of several volts to over ten volts applied to the photoelectric transfer film at the maximum.
On the other hand, in the direct conversion system, the X-ray-to-charge converting film, the pixel capacity and the TFT serving as a switch for each pixel are connected in series, and a high bias of several kV is applied thereto. Therefore, when X-rays are incident on the pixel, the electric charge produced by the X-ray-to-charge converting film accumulates in the pixel capacity. However, if the quantity of incident X-ray is excessive, the electric charge accumulating in the pixel capacity increases, so that it is afraid that a high voltage of more than 10 kV is applied to the insulator films of the pixel capacity and the TFT to cause electrical break-down. For that reason, the direct conversion system must take measures to prevent an excessive voltage from being applied to the pixel capacity and TFTS.
Therefore, a protecting TFT serving as a protecting non-linear element is provided in each of pixels. Thus, when excessive X-rays enter a pixel, a higher electric charge than that defined by a bias is. discharged to the outside of the pixel via the protecting TFT to prevent the dielectric breakdown of the TFT and pixel capacity.
FIG. 9 shows the construction of a pixel of a flat-panel X-ray detector of a direct conversion system using the protecting TFT, and the operation thereof will be described below.
Each pixel 801 of a flat-panel X-ray detector of a direct conversion system shown in FIG. 9 comprises a TFT 701 used as a switching element, an X-ray-to-charge converting film 802, and a pixel capacity 703. Similar to the X-ray detector shown in FIG. 8, the pixels 801 are arranged in the form of an array. The pixel capacity 703 is connected to a pixel capacity bias 803. To the X-ray-to-charge converting film 802, a negative bias voltage is applied by a high-voltage power supply 804. The gate of the TFT 701 is connected to a scanning line 706, and one of the source and drain of the TFT 701 is connected to a signal line 705, so that the on/off of the TFT 701 is controlled by means of a scanning line driving circuit 707. The terminal of the signal line 705 is connected to an amplifier 710 for signal detection. A protecting TFT 805 is biased by a power supply 807 via a bias line 806. The protecting TFT 805 allows an electric charge of a bias voltage or higher to escape through the bias line 806.
In both of the X-ray image detector systems using the flat-panel X-ray detectors of the direct and indirect conversion systems using the TFTs 701, very weak signals can not be detected, so that there is a lower limit to the X-ray irradiation intensity to a human body. Because a signal voltage shift is produced by a floating capacity which is received by the signal line 705 from the intersecting scanning line 706 and the bias line 806, and because there is a limit to the reduction of noises of the amplifier 710 for signal detection and noises due to leak currents or the like of the protecting TFT 805 used for the flat-panel X-ray detector of the direct conversion system. In order to solve this problem, it is considered that an amplifier circuit for amplifying the electric charge produced in the photoelectric transfer film 702 or X-ray-to-charge converting film 802 is provided for each pixel 801. However, it is difficult to realize this since the amplifier circuit is prepared by the same design rule as that of the TFTs for pixels to cause the area of only the amplifier circuit to be greater than the pixel area.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide an X-ray detector system capable of picking up an image even if X-ray irradiation is weak.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, an X-ray image detector system comprises: a plurality of scanning lines; a plurality of signal lines formed so as to intersect the scanning lines; and a plurality of pixel parts, each of which is formed at a corresponding one of the intersections between the scanning lines and the signal lines so as to form an array; each of said pixel parts having an X-ray-to-charge converting part for converting an incident X-ray to an electric charge, a pixel electrode for receiving the electric charge from the X-ray-to-charge converting part, and a switching element which is operated on the basis of a signal of a corresponding one of the scanning lines, one end of the switching element being connected to the pixel electrode, and the other end of the switching element being connected to a corresponding one of the signal lines, wherein the X-ray-to-charge converting part includes at least a first X-ray-to-charge converting film, and a second X-ray-to-charge converting film having a lower resistivity than that of the first X-ray-to-charge converting film.
Furthermore, an electric field for causing a current multiplication is preferably applied to the first X-ray-to-charge converting film.
In addition, each of the first and second X-ray-to-charge converting films is preferably formed of Se, and the electric field for causing the current multiplication is preferably 9xc3x97107 V/m.
The X-ray-to-charge converting part may have a third X-ray-to-charge converting film of a first conductive type formed between the pixel electrode and the first X-ray-to-charge converting film, and a fourth X-ray-to-charge converting film of a second conductive type formed on the second X-ray-to-charge converting film, the second X-ray-to-charge converting film being formed on the first X-ray-to-charge converting film.
The X-ray-to-charge converting part may have a fifth X-ray-to-charge converting film having a lower resistivity than that of the first X-ray-to-charge converting film between the third X-ray-to-charge converting film and the first X-ray-to-charge converting film. For example, if the X-ray-to-charge converting film is formed of Se, each of the first, second and fifth X-ray-to-charge converting films often have a high resistivity, and is often formed of an i-type (intrinsic) semiconductor containing no intentionally doped impurities decreasing resistivity, or an i-type semiconductor containing a small amount of impurity. On the other hand, the third and fourth X-ray-to-charge converting films are often formed of an n-type semiconductor containing a large amount of donor impurity, or a p-type semiconductor containing a large amount of acceptor impurity. The third and fourth X-ray-to-charge converting films can decrease the resistivity to the upper or lower metal electrode to form an ohmic contact and can decrease the dark current during no X-ray irradiation serving as a noise source. Therefore, the third and fourth X-ray-to-charge converting films have the function of preventing majority carrier from being introduced from the electrode. The third and fourth X-ray-to-charge converting films may be formed of any materials of these effects.
Furthermore, at least one boundary surface of the first X-ray-to-charge converting film is preferably flattened.
In addition, the first X-ray-to-charge converting film preferably have a non-flat portion on the boundary surface, the non-flat portion being filled with a conductive material.
Moreover, the thickness of the fifth X-ray-to-charge converting film is preferably smaller than the thickness of the second X-ray-to-charge converting film.
According to another aspect of the present invention, an X-ray image detector system comprises: a plurality of scanning lines; a plurality of signal lines formed so as to intersect the scanning lines; a plurality of pixel parts, each of which is formed at a corresponding one of the intersections between the scanning lines and the signal lines so as to form an array; each of said pixel parts having an X-ray-to-luminescent light converting part for converting an incident X-ray to luminescent light, a photoelectric transfer part, formed on the plurality of pixel parts, for converting the luminescent light, which is converted by the X-ray-to-luminescent light converting part, to an electric charge, a pixel electrode for receiving the electric charge from the photoelectric transfer part, and a switching element which is operated on the basis of a signal of a corresponding one of the scanning lines, one end of the switching element being connected to the pixel electrode, and the other end of the switching element being connected to a corresponding one of the signal lines, wherein the photoelectric transfer part includes at least a first photoelectric transfer film, and a second photoelectric transfer film having a lower resistivity than that of the first photoelectric transfer film.